The embodiments described herein relate generally to operating a power generation system, and more specifically, to using data obtained from a phasor measurement unit (PMU) to control the power generation system.
Solar and wind are examples of renewable sources of energy that are increasingly becoming attractive alternative sources of energy. Solar energy in the form of sunlight may be converted to electrical energy by solar cells. A more general term for devices that convert light directly into electrical energy is “photovoltaic cells.” Solar farms include a plurality of solar cells coupled together to provide a desired level of power. Wind energy may be converted to electrical energy using a wind turbine generator. Wind turbine generators typically include a rotor having multiple blades that transform wind energy into rotational motion of a drive shaft, which in turn is utilized to drive an electric generator. Wind farms include a plurality of wind turbine generators coupled together to provide a desired level of power.
Power generated by an electric utility, using renewable sources of energy or fossil fuel based sources of energy, is typically delivered to a customer over an electrical grid. Generated power may be provided to power electronics, for example, a power converter, for conditioning of the power prior to application to the electrical grid. Electricity applied to the electrical grid is required to meet grid connectivity expectations. These requirements address safety issues as well as power quality concerns. For example, the grid connectivity expectations include operating the power generation system during a transient grid event, for example, a short circuit fault on the electrical grid. This capability may be referred to as low voltage ride through (LVRT) or zero voltage ride through (ZVRT). An LVRT/ZVRT event is a condition where the alternating current (AC) utility voltage is low on either one phase of the electrical grid or multiple phases of the electrical grid. During an LVRT/ZVRT event, the capacity of the electrical grid to accept power from the power generation system is low. Another grid connectivity expectation is that the generation system outputs match the voltage and frequency of the electricity flowing through the electrical grid.
The remote location and high power rating of typical renewable energy farms typically cause a weak grid connection unless high-value investment is applied to enhance the connection, for example, through inclusion of larger transmission lines and/or circuit compensation equipment. A low short-circuit ratio (SCR) is an indication that the grid is weak. For example, when the SCR is below 1.5, a traditional current controlled type of generator may not be able to operate stably and a voltage controlled type of generator has to be tuned relatively slow to operate stably. To operate the generator with the slow control an additional hardware investment is needed to assure enough operation margin (e.g., the size of a DC link capacitor included within the power converter may need to increase).
Furthermore, typically, electric generators having high power ratings are included within a renewable energy farm to allow the farm to ride through a fault event and avoid post-fault load shedding or blackout. However, during a low-voltage and especially a zero-voltage fault event, keeping synchronized with the grid voltage is difficult because the measured grid reference voltage typically used to maintain synchronization between an electric generator and the grid is low. If a fault event is severe, there may not be enough residual grid voltage for a controller to use as a reference. In this circumstance, the generator controller may independently determine a frequency at which to output power, which may be out of step with the grid. If during a recovery from a fault event the generators are out of step with the grid, a detrimentally large transient current may occur due to the phase angle difference between the generators and the grid.